Supercritical Flow

A total of 49 extended abstracts are included in this volume of papers presented at the 15th International Workshop on Water Waves and Floating Bodies, held 17 February - 1 March, 2000, Caesarea, Israel. Also see http://www.eng.tau.ac.il/-miloh/iwwwfb. The objective of the Workshop is to provide a forum for informal discussions of fundamental research, of mutual interest to both engineers and scientists, in the broad area of wave interactions with fixed, floating or submerged bodies.

1] The classical hydraulic jump, which is a hydraulic jump formed in a horizontal, smooth, and prismatic laboratory flume, has often been used for estimating hydraulic and morphologic characteristics of mountain river channels. However, most studies utilizing classical hydraulic jump relationships have neglected the role of aerated flows and jump types prevalent in natural systems. By developing a methodology to survey, digitize, and visualize air concentrations for jumps in bedrock rivers, we present new data for submerged jumps and sloping jumps in a bedrock step-pool channel. As with laboratory research, our results indicated peak, mean, and local air concentrations of natural jumps were skewed by submergence of the jump roller beneath the free surface. However, unlike laboratory research, cross-stream variations were significant for both jumps, varying up to 20% across the jump width. Traces through each jump thalweg also indicated the classical hydraulic jump was a poor analogue for estimating surficial air concentrations.

Turbidity currents in the ocean are driven by suspended sediment. Yet results from surveys of the modern sea floor and turbidite outcrops indicate that they are capable of transporting as bedload and depositing particles as coarse as cobble sizes. While bedload cannot drive turbidity currents, it can strongly influence the nature of the deposits they emplace. This paper reports on the first set of experiments which focus on bedload transport of granular material by density underflows. These underflows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud which is so fine that it does not settle out readily. Thus, all the currents can be considered to be model turbidity currents. The data cover four bed conditions: plane bed, dunes, upstream-migrating antidunes and downstream-migrating antidunes. The bedload transport relation obtained from the data is very similar to those obtained for openchannel flows and, in fact, is fitted well by an existing relation determined for open-channel flows. In the case of dunes and downstream-migrating antidunes, for which flow separation on the lee sides was observed, form drag falls in a range that is similar to that due to dunes in sand-bed rivers. This form drag can be removed from the total bed shear stress using an existing relation developed for rivers. Once this form drag is subtracted, the bedload data for these cases collapse to follow the same relation as for plane beds and upstream-migrating antidunes, for which no flow separation was observed. A relation for flow resistance developed for open-channel flows agrees well with the data when adapted to density underflows. Comparison of the data with a regime diagram for field-scale sand-bed rivers at bankfull flow and field-scale measurements of turbidity currents at Monterey Submarine Canyon, together with Shields number and densimetric Froude number similarity analyses, provide strong evidence that the experimental relations apply at field scale as well.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. coaxial-jet injector similar to those used in cryogenic liquid rockets, a non-reacting-flow experimental investigation was conducted under sub-, near-, and supercritical chamber pressures, with and without acoustical excitation. Past research works on this subject have shown both the relevance and importance of geometrical changes in an injector's exit-area and its nearby physical and fluid mechanical processes. On this basis, special attention is paid in collecting spatiallyresolved mean temperatures and documenting the aforementioned interactions at the exit of this injector. Short-duration and high-speed digital cameras provided information on the dynamic behavior of this jet under a variety of conditions. Mean and root mean square (RMS) values of the coaxial-jet dark-core length fluctuations were measured from the acquired images via a computer-automated method. It is seen that as the outer-to-inner jet velocity ratio increases, the RMS of the dark-core length fluctuations decreases. It is hypothesized that a connection to rocket instability can be obtained from these data through examination of the RMS values of the dark-core length fluctuations. It is possible that decreases in the fluctuation levels, which were shown here to occur at higher velocity ratios, could weaken a key feedback mechanism for the self-excitation process that is believed to drive the combustion instability in rocket engines. This could offer a possible explanation of the combustion stability improvements experienced in engines under higher outer-to-inner jet velocity ratios. Additional analysis and data acquisition are planned to further investigate this finding. Finally, after a careful review of relevant data taken here and by others, there appears to be a good correlation between the dark-core length and the outer-to-inner jet momentum flux ratio, but the form of this dependency was found to be different at subcritical pressures than the rest of the chamber conditions (i.e., near-and supercritical cases).

Hydraulic data defining the dune:antidune transition in fine gravel are compared with potential flow theory, and information is drawn from published experiments and field-based studies. Attention is given to both transitional bedforms and the development of downstream-migrating antidunes. In the latter case, most data pertain to sand beds and not to gravel. Empirical data provide some weak support for the theoretical notion that the transition occurs at progressively lower Froude numbers at greater relative depths. Although a critical Froude number of 0AE84 may reasonably be applied for the beginning of the dune to antidune transformation, lag effects (and a possible depth limitation) ensure that transitional bedforms may persist across a broad range of Froude numbers from 0AE5 to 1AE8. This latter observation has great relevance for palaeohydraulic estimates derived from outcrop data. Whereas the application of theoretical bedform existence fields, based upon potential flow theory, to fine gravel was previously purely speculative, the addition of experimental and field data to these plots provides a degree of confidence in applying stability theory to practical geological problems. For the first time, laboratory data pertaining to downstream-migrating gravel antidunes are compared with theory. These bedforms have been reported from certain experimental near-critical flows above sand or gravel beds, but have been observed infrequently in natural streams. However, there are no detailed studies from natural rivers and only a few contentious identifications from outcrops. Nevertheless, the limited hydraulic data conform to theoretical expectations.

We describe here the cyclic voltammetry (CV) and chronoamperometry of Fe(bpy)l* (bpy = 2,2'-bipyridine) in SO, at temperatures up to 174" C, above the critical point of SO2 (T, = 158O C, PC = 77.8 atm), and discuss the effect of temperature on the diffusion coefficient and the electrochemical behavior. Near-critical and supercritical fluids are of interest because the solvent properties are very different from those of the liquid form and are adjustable over a wide range by variation of the temperature and pressure. Recent studies have demonstrated that electrochemical experiments can be carried out in supercritical H,O [l], NH, [2], CO2 + H,O [3] and MeCN [4]

We have investigated shallow water flows through a channel with a contraction by experimental and theoretical means. The horizontal channel consists of a sluice gate and an upstream channel of constant width b 0 ending in a linear contraction of minimum width b c . Experimentally, we observe upstream steady and moving bores/shocks, and oblique waves in the contraction, as single and multiple ͑steady͒ states, as well as a steady reservoir with a complex hydraulic jump in the contraction occurring in a small section of the b c / b 0 and Froude number parameter plane. One-dimensional hydraulic theory provides a comprehensive leading-order approximation, in which a turbulent frictional parametrization is used to achieve quantitative agreement. An analytical and numerical analysis is given for two-dimensional supercritical shallow water flows. It shows that the one-dimensional hydraulic analysis for inviscid flows away from hydraulic jumps holds surprisingly well, even though the two-dimensional oblique hydraulic jump patterns can show large variations across the contraction channel.

This is a review of the Hydrologic Criteria and Drainage Design Manual (Clark County Regional Flood Control District 1999) of the Clark County Regional Flood Control District (CCRFCD). Areas of needed research relative to channels conveying supercritical flow and corresponding channel appurtenances have been identified.

Bedforms and associated sedimentary structures, formed under supercritical water¯ow over an aggrading sand bed, were studied in a laboratory¯ume. Although the geometry and hydraulic characteristics of these bedforms (antidunes, chutes-and-pools) are well known, their internal structures are not. The objectives of the study were to: (1) describe the three-dimensional geometry of the sedimentary structures and examine their mode of origin; (2) develop a relationship between the geometries of the sedimentary structures and the formative bedforms and; (3) identify criteria that distinguish these sedimentary structures from similar types, such as hummocky and swaley cross-strata. Sedimentary structures associated with antidunes are primarily lenticular laminasets with concave-upward erosional bases (troughs) in which laminae generally dip upstream or ®ll the troughs symmetrically. These laminasets are associated with growth and upstream migration of water-surface waves and antidunes, and with surface-wave breaking and ®lling of antidune troughs respectively. In addition, sets of downstream-dipping laminae are produced by rapid migration of asymmetrical bedwaves immediately after wave breaking. Rare convex-upward laminae de®ne the shape of antidunes that developed under stationary water-surface waves. The laminasets and internal laminae extend across the width of the¯ume, but vary in thickness and inclination, indicating that the antidunes have some degree of three dimensionality. The length and maximum thickness of the lenticular laminasets are approximately half of the length and height of formative antidunes, providing a potentially useful tool for palaeohydraulic reconstructions. The sets of downstream-dipping laminae formed under antidunes are distinctive and do not occur in hummocky and swaley cross-strata. Sedimentary structures associated with chutesand-pools are sets of upstream-dipping laminae and structureless sand.

A series of laboratory experiments in a 6 m long chute using glass particles of mean diameter 100 μm were performed to investigate the interaction of a supercritical, granular flow with obstacles. It was found that the collision of the flow with a row of mounds led to the formation of a jet, whereby a large fraction of the flow was launched from the top of the mounds and subsequently landed back on the chute. The retarding effect of the mounds was investigated quantitatively by direct measurements of the velocity of the flow, its runout length and the geometry of the jet. The effects of several aspects of the layout of the mounds on their retarding effects were examined. It was observed that a row of steep mounds with an elongated shape in the transverse direction to the flow and with a height several times the flow depth led to dissipation of a large proportion of the kinetic energy of the flow.

Turbidity currents in the ocean and lakes are driven by suspended sediment. The vertical profiles of velocity and excess density are shaped by the interaction between the current and the bed as well as between the current and the ambient water above. We present results of a set of 74 experiments that focus on the characteristics of velocity and fractional excess density profiles of saline density and turbidity currents flowing over a mobile bed. The gravity flows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud that is so fine that it does not settle out readily. Thus, all the currents can be considered to be model turbidity currents. The data cover both Froude-subcritical and Froude-supercritical regimes. Depending on flow conditions, the bed remains flat or bed forms develop over time, which in turn affect vertical profiles. For plane bed experiments, subcritical flow profiles have velocity peaks located higher up in the flow, and display a sharper interface at the top of the current, than their supercritical counterparts. The latter have excess density profiles that decline exponentially upward from the bed, whereas subcritical flows show profiles with a region near the bed where excess density varies little. Wherever bed forms are present, they have a significant effect on the profiles. Especially for Froude-supercritical flow, bed forms push the location of peak velocity upward, and render the near-bed fractional excess density more uniform. In the case of subcritical flow, bed forms do not significantly affect fractional excess density profiles; velocity profiles are pushed farther upward from the bed than in the case of a plane bed, but to a lesser extent than for supercritical bed forms. Overall, the relative position of the velocity peak above the bed shows a dependence upon flow regime, being lowered for increasing Froude number F d . Gradient Richardson numbers Ri g in the near-bed region increase with increasing F d , but are lower than the critical value of 0.25, indicating that near-bed turbulent structures are not notably suppressed. At the top interface, values of Ri g are above the critical value for subcritical and mildly supercritical F d , effectively damping turbulence. However as F d increases, Ri g goes below the critical value. Shape factors calculated from the profiles for use in the depth-averaged equation of motion are evaluated for different flow and bed conditions. Normalized experimental profiles for supercritical currents scale up well with observations of field-scale turbidity currents in the Monterey Canyon, and the range of average bed slopes and Froude numbers also compare favorably with estimated field-scale flow conditions for the Amazon canyon and fan. This suggests that the experimental results can be used to interpret the kinds of flows that are responsible for the shaping of major submarine canyon-fan systems.

Recent experience gained at AFRL with the injection of cryogenic fluids into high back-pressures is summarized. In the experimental investigations described, a jet of a cryogenic fluid, typically liquid N2, is injected into a chamber whose ambient pressure is varied to values exceeding the critical pressure of the injectant. The structure of the jet and the shear layer between the jet and the ambient has been examined. Results from visualization, jet growth rate, fractal analysis, and Raman scattering measurements indicate that the behavior of the injected fluid changes from spray-like behavior to gas jet-like behavior as pressure increased.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time f maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Inforr Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM lo TMt ABOVE ADDRESS.

A series of laboratory experiments in a 6 m long chute using glass particles of mean diameter 100 μm were performed to investigate the interaction of a supercritical, granular flow with obstacles. It was found that the collision of the flow with a row of mounds led to the formation of a jet, whereby a large fraction of the flow was launched from the top of the mounds and subsequently landed back on the chute. The retarding effect of the mounds was investigated quantitatively by direct measurements of the velocity of the flow, its runout length and the geometry of the jet. The effects of several aspects of the layout of the mounds on their retarding effects were examined. It was observed that a row of steep mounds with an elongated shape in the transverse direction to the flow and with a height several times the flow depth led to dissipation of a large proportion of the kinetic energy of the flow.

Heat transfer in upward flows of supercritical water in circular tubes and in tight fuel rod bundles is numerically investigated by using the commercial CFD code STAR-CD 3.24. The objective is to have more understandings about the phenomena happening in supercritical water and for designs of supercritical water cooled reactors. Some turbulence models are selected to carry out numerical simulations and the results are compared with experimental data and other correlations to find suitable models to predict heat transfer in supercritical water. The comparisons are not only in the low bulk temperature region, but also in the high bulk temperature region. The two-layer model (Hassid and Poreh) gives a better prediction to the heat transfer than other models, and the standard k-ε high Re model with the standard wall function also shows an acceptable predicting capability. Three-dimensional simulations are carried out in sub-channels of tight square lattice and triangular lattice fuel rod bundles at supercritical pressure. Results show that there is a strong non-uniformity of the circumferential distribution of the cladding surface temperature, in the square lattice bundle with a small pitch-to-diameter ratio (P/D). However, it does not occur in the triangular lattice bundle with a small P/D. It is found that this phenomenon is caused by the large non-uniformity of the flow area in the cross-section of sub-channels. Some improved designs are numerically studied and proved to be effective to avoid the large circumferential temperature gradient at the cladding surface.

We are reporting a novel synthetic approach to prepare organic modified Ba-hexaferrite nanocrystals with controlled size and morphology under a supercritical water (SCW) flow reactor process. The surface of the Ba-hexaferrite nanocrystal was capped with oleic acid ligand to control the particle growth, size, and morphology. The results showed that the concentration of oleic acid reagent played a key role in controlling the cubic and octahedral shape of Ba-hexaferrite nanocrystals. This indicates that the size and morphology of nanocrystals were greatly influenced by the organic modification under the SCW process, resulting in a drastic reduction in the particle size from 30 to 9 nm. The organic ligand capped Ba-hexaferrite nanocrystals exhibit a higher coercivity (about 2800 Oe) at 5 and 280 K temperature. This study provides a novel approach for large-scale production of complex metal oxide colloidal nanocrystals with uniform size, well-defined shape, and controlled surface chemistry.

A new algorithm for the solution of the shallow water equations is introduced. The formulation is founded on a suitable operator-splitting procedure for which a characteristic-based rational form of including balancing dissipation terms is achieved.In the semi-explicit form the method circumvents the requirement of a critical time step given in terms of the wave celerity, which is restrictive for the analysis of long-wave propagation in shallow waters.In this work the robustness of the algorithm is illustrated for transient shallow water problems and for some supercritical flows, where the choice of an algorithm with optimal diffusion properties is manifest.

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources. gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to the Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

Ternary phase equilibrium data are presented for acetic acid-water mixtures with supercritical carbon dioxide at temperatures of 313 and 323 K and pressures of 73, 104 and 139 bar. Selectivities and distribution coefficients were obtained from these data and are also reported. Results ...

This paper presents a coastal hydrodynamic model for simulating coastal barrier breaching flows through an inlet which are mostly induced by extreme hydrological conditions such as storm/hurricane surges, waves and tides. In order to simulate wave field and wave-induced flow field in a coast, a wave action spectral model is coupled with a hydrodynamic model. The Godunov-type shock-capturing technique is used in the hydrodynamic model to simulate the supercritical flows and shocks driven by the extreme storm conditions. The hydrodynamic model is based on the solution of depth-averaged non-linear shallow water equations with all physical forcings common to coastal hydrological conditions so that it is capable of simulating multiple flow regimes, in which subcritical, transcritical, or supercritical flows may happen. The bed slope terms in the system of equations are treated in such a way that exact balance between flux gradient and bed slope terms is achieved under still water condition. The wave model readily provides the radiation stresses that represent the shortwave-averaged forcings in a water column and take into account wave-induced nearshore currents. In the coupled system, the models are operated systematically. The coastal hydrodynamic model is shown to accurately reproduce analytical and benchmark numerical solutions. To further test the accuracy of the model, flow through a coastal inlet with a storm surge is simulated and the results are compared with an established coastal flow model. Finally, the model is examined to simulate a severe storm surge that develops supercritical flows and the results are found to be encouraging.

A non-reacting-flow experimental investigation was undertaken to gain a better understanding of some of the underlying physics associated with the interaction of acoustic waves and a coaxial-jet injector similar to those used in cryogenic liquid rockets. Liquid nitrogen (the round inner jet) and gaseous nitrogen (the annu-lar outer jet) were used under subcritical, near critical, and supercritical chamber pressures, with and with-out acoustic excitation. High-speed digital imaging provided information on the dynamic behavior of the jet under a variety of conditions. It is found that when the jet is at the pressure node, an externally-imposed acoustic field excites the dark-core of the jet to a wavy-shaped structure consistent with the field's charac-teristics. Mean and root mean square (RMS) values of the dark-core length fluctuations were measured from images. It is seen that as the outer-to-inner-jet velocity ratio increases, the RMS of the dark-core length fluctuations decrea...

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. supercritical pressures is presented. Such fields and their interaction with the jets (i.e., breakup, mixing, etc.) are believed to play a critical role during combustion instabilities in liquid rocket engines. The shear coaxial injector used here is similar to those used in cryogenic liquid rockets. By using N 2 as the working fluid, the chemistry effects on combustion instability are separated from the effects of a transverse acoustic field on coaxial jets. Furthermore, through this choice, ambiguities associated with composition dependence on mixtures critical properties are eliminated. The acoustic field is generated by a piezo-siren and the first resonant frequency is ~3kHz. The pressures in the chamber range from 215-716 psia to span subcritical to supercritical pressures. The outer to inner jet velocity ratio varies from ~1.2 to 23 and the momentum flux ratio (MR) varies from ~0.2 to 23. These ratios are mainly varied by changing the temperature and flow rates of the outer jet. At least 2000 backlit images were taken at 41kHz for each run. The main metric investigated is the length of the dark, or inner jet, core. This length is related to the mixing processes in a coaxial jet. The shorter the core length the faster the mixing occurs. Both the axial and the total, or curved, dark core lengths are studied. For momentum flux ratios ~1<MR<~4 the differences in the axial and curved dark core lengths between acoustics on and off are statistically significant, which means acoustics do shorten the core for this range. For subcritical pressures the MR range where the jet is shortened is larger. Preliminary results on the frequency analysis of the dark core lengths and width is also presented. Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified SAR 18 19b. TELEPHONE NUMBER (include area code) N/A Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. 239.18 1 Sr. Aerospace Engineer,

This technical note studies the supercritical junction flow occurring at the right-angled confluence of four equal-width channels in which two upstream channels carry flow towards the junction. The note seeks to investigate, firstly, the pertinence of a 2D shallow water equation model to reproduce the typical flow structures at the junction. The second objective is to build a computationally efficient model with more resolution applied at critical point i.e. junction and less in areas where the flow is primarily 1D i.e. channels and compare such model, named, ‘sparse’ with a uniformly meshed model regarding solution accuracy and computational efficiency. The results indicate that the sparse model is able to reproduce typical flow structures appearing at the channel junction in an adequate manner. The discharge distribution is fairly well predicted. The jump angles are almost the same in the two models as well as the location and size of the recirculation zones and the flow depth super-elevation areas. However, the two models diverge in the prediction of very small depths in the recirculation zone where the sparse model overestimates the depths. As regards, computational efficiency, the sparse model is found to be 61% more efficient than the uniform mesh model.

Impact forces by snow avalanches on narrow obstacles need to be taken into account in the design of many constructions in avalanche prone terrain, such as masts of electrical power lines, ski lifts and cable cars. An important question in connection with such impact forces on high obstacles that extend through the flow is how they depend on the width and cross-sectional shape of the obstacle for a given velocity and thickness of the oncoming flow. Widely used engineering guidelines imply that a significant fraction of the dynamic pressure of the avalanche impacts the obstacle simultaneously over a substantial part of the full height range corresponding to the run-up of the avalanche. A series of laboratory experiments with granular material was conducted in a 7.5 m long and 0.35 m wide chute in order to investigate impact forces on narrow rectangular and cylindrical obstacles for rapid supercritical granular flow (Fr = 13). Obstacle heights varied from about twice the flow depth to more than 20 times the flow depth, which was higher than the observed run-up, and the width of the obstacles varied from about twice the flow depth to about 7 times the flow depth. It was found that the total force on the obstacle was largely unrelated to the obstacle height for heights that exceeded about 3 flow depths, which is much lower than the run-up on the highest obstacles. For a wide range of obstacle heights exceeding about 3 flow depths, the total force on the rectangular obstacles was of similar magnitude to the dynamic pressure, 1 2 q f u 2 ; acting over an area corresponding to the width of the obstacle and the upstream depth of the flow. The total force was about 30% lower for cylindrical compared with rectangular obstacles. The variation of impact forces on obstacles with the flow velocity for thin, free-surface granular flows may be significantly different from confined flows where a free surface is not present. The presence of the free surface may lead to much lower impact forces at very high velocity than would be expected by comparison with confined flows.

Se estudiaron alternativas de diseño para la rápida curva de un aliviadero mediante la complementación entre modelación física y matemática, en la que el principal fenómeno de análisis fueron las ondas cruzadas generadas bajo el régimen de escurrimiento supercrítico. Pueden distinguirse dos estados de funcionamiento para el aliviadero; si el caudal es menor a 220 m 3 /s, la erogación se produce a través de un vertedero de perfil que descarga frontalmente sobre una rápida de pendiente variable y desarrollo curvo; por encima de este caudal entra en funcionamiento un vertedero lateral de perfil triangular, lo cual altera el las características de aproximación del flujo a la rápida. Inicialmente se estudió el diseño original en modelo físico. Debido a la formación de las ondas cruzadas, los niveles del pelo de agua en las paredes de la rápida no resultaban admisibles por generar desborde. Es por ello que se evaluaron dos alternativas con distintos grados de peralte en la curva y modific...

We describe here the cyclic voltammetry (CV) and chronoamperometry of Fe(bpy)l* (bpy = 2,2'-bipyridine) in SO, at temperatures up to 174" C, above the critical point of SO2 (T, = 158O C, PC = 77.8 atm), and discuss the effect of temperature on the diffusion coefficient and the electrochemical behavior. Near-critical and supercritical fluids are of interest because the solvent properties are very different from those of the liquid form and are adjustable over a wide range by variation of the temperature and pressure. Recent studies have demonstrated that electrochemical experiments can be carried out in supercritical H,O [l], NH, [2], CO2 + H,O [3] and MeCN [4]

The weakly nonlinear dynamics of long waves embedded in marginally stable shear flows that vary in the streamwise direction are shown to be governed by a variable-coefficient Boussinesq equation. Depending on the local stability characteristics of the flow, new nonmodal or modal instabilities may emerge that serve as natural mechanisms for achieving amplitude thresholds necessary for weakly nonlinear instability and the transition to fully finite-amplitude states.

pressures up to 100 atm, and residence times up to 550 sec. Analysis, by HPLC/UV, of the toluene reaction products led to the unequivocal identification of 27 individual PAH, up to 10 rings in size. The experiments showed that PAH yields increased exponentially with pressure. For each of the product PAH, values of the preexponential factor A and the activation volume V* were determined for the pressure-dependent global first-order kinetic rate constant. PAH yields were also extremely sensitive to temperature. At 100 atm and 480 C, PAH of only up to 2 rings were observed; at 535 C, PAH up to 10 rings were formed; at 585 C, solid deposits formed, plugging the reactor. Similar pressure and temperature sensitivities were exhibited by the PAH produced by methylcyclohexane. The nature of the PAH product distributions suggested that condensation reactions, involving the addition of aromatic structures and hydrogen loss, were a dominant mechanism for PAH formation in the supercritical environment. 15. SUBJECT TERMS supercritical hydrocarbon pyrolysis, polycyclic aromatic hydrocarbons, endothermic fuels, fliel-line deposits, hypersonic aircraft fuels, reaction kinetics, PAH formation, pressure effects 16. SECURITY CLASSIFICATION OF: a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACT UL 18. NUMBER OF PAGES 66 198. NAME OF RESPONSIBLE PERSON Julian M. Tishkoff 19b. TELEPHONE NUMBER {Include area code) (703)696-8478 k\moi> Standard Form 298 (Rev. 8/98) ftesoribed by ANSI Std. Z39.18 Background and Introduction The fuels used in the next generation of hypersonic aircraft will have to operate under very high pressures and will have to sustain very high heat loads (e.g., 30,000 BTU/min) [1,2]) in order to meet aircraft cooling requirements. Within the fuel lines and injection system, where residence times can be many minutes, fuel temperatures and pressures may reach or exceed 540 °C (813 K) and 150 atm [2]-temperatures and pressures that exceed the critical temperatures and pressures of most pure hydrocarbons and jet fuels such as JP-7 and JP-8 [3]. At these temperatures and pressures, the fuel can undergo pyrolytic reactions, which have the potential of forming solid deposits that can clog fuel lines, foul fuel nozzles, and lead to undesirable or even disastrous effects for the akcraft. The current inability to predict solids formation tendencies of fuels under supercritical conditions has been brought to our attention by Dr. Tim Edwards (Fuels Section, AFRL Turbine Engine Division, Wright-Patterson Air Force Base, Ohio), who shared with us solid deposition results for various jet ftiels and n-octane, from scramjet test rigs at United Technologies Research Center (UTRC) [4]

: An experimental study on the effects of an externally-imposed transverse acoustic field in a N2 shear coaxial jets at sub-, near-, and supercritical pressures is presented. Such fields and their interaction with the jets (i.e., breakup, mixing, etc.) are believed to play a critical role during combustion instabilities in liquid rocket engines. The shear coaxial injector used here is similar to those used in cryogenic liquid rockets. By using N2 as the working fluid, the chemistry effects on combustion instability are separated from the effects of a transverse acoustic field on coaxial jets. Furthermore, through this choice, ambiguities associated with composition dependence on mixtures critical properties are eliminated. The acoustic field is generated by a piezo-siren and the first resonant frequency is ~3kHz. The pressures in the chamber range from 215-716 psia to span subcritical to supercritical pressures. The outer to inner jet velocity ratio varies from ~1.2 to 23 and the mo...

A system response matrix method, which directly solves the linearized differential equations in the matrix form without Laplace transformation, is introduced for the supercritical fluids flow instability analysis. The model is developed and applied to the single channel or parallel channel type instability analyses of a typical proposed Supercritical Water Reactor (SCWR) design. A uniform axial heat flux is assumed, and the dynamics of the fuel rods and water rods are not considered in this paper. The sensitivity of the decay ratio (DR) to the axial mesh size is analyzed and found that the DR is not sensitive to mesh size once sufficient number of axial nodes is applied. The sensitivity of the stability to inlet orifice coefficient is conducted for the hot channel and found that a higher inlet orifice coefficient will make the system more stable. The susceptibility of stability to operating parameters such as mass flow rate, power and system pressure is also performed. It is found that the SCWR stability sensitivity feature can be improved by carefully choosing the inlet orifice coefficients and operating parameters. The stability feature of the average channel is also analyzed with an equivalent inlet orifice coefficient. Finally, the manufacturing feasibility of the inlet orifices for both the hot channel and average channel is studied and found to be favorable.

A series of laboratory experiments in a 6 m long chute using glass particles of mean diameter 100 mum were performed to investigate the interaction of a supercritical, granular flow with obstacles. It was found that the collision of the flow with a row of mounds led to the formation of a jet, whereby a large fraction of the flow

A fundamental problem in avalanche engineering is to determine the impact pressures exerted on structures. This task is complicated because snow avalanches flow in a variety of regimes, primarily depending on snow temperature and moisture content. In this paper we address this problem by analyzing measured impact pressures, flow velocities, and flow depths of five Vallée de la Sionne avalanches. The measurements are made on a 20 m high tubular pylon instrumented with high-frequency pressure transducers and optoelectronic velocity sensors. In the observed avalanches, we find both subcritical and supercritical flow regimes. Typical Froude numbers were smaller than 6. The subcritical regime (Fr < 1) is characterized by a flow plug riding above a highly sheared basal layer. The measured pressures are large and velocity-independent in contradiction to calculation procedures. Pressure fluctuations increase with flow depth, indicating a kinematic stick-slip phenomena which is largest at the basal layer. Supercritical flow regimes (1 < Fr < 6) are characterized by a sheared flow all over the avalanche depth. In this regime the impact pressure is velocity-dependent. We derive relationships governing impact pressure as a function of the Froude number, and therefore flow regime, encompassing all the observed avalanches.

We present estimates of internal shear rates of real-scale avalanches that are based on velocity measurements. Optical velocity sensors installed on the instrument pylon at the Swiss Vallée de la Sionne test site are used to measure flow velocities at different flow heights of three large dry and wet snow avalanches. Possible sources of error in the correlation analysis of the time-lagged reflectivity signals measured by optical sensors are identified for real-size avalanches. These include spurious velocities due to noise and elongated peaks. An appropriate choice of the correlation length is essential for obtaining good velocity estimates. Placing restrictions on the maximum possible accelerations in the flow improves the analysis of the measured data. Coherent signals are found only in the dense flowing cores. We observe the evolution of shear rates at different depths between the front and tail of the flowing avalanche. At the front, large shear rates are found throughout the de...

Impact forces by snow avalanches on narrow obstacles need to be taken into account in the design of many constructions in avalanche prone terrain, such as masts of electrical power lines, ski lifts and cable cars. An important question in connection with such impact forces on high obstacles that extend through the flow is how they depend on the width and cross-sectional shape of the obstacle for a given velocity and thickness of the oncoming flow. Widely used engineering guidelines imply that a significant fraction of the dynamic pressure of the avalanche impacts the obstacle simultaneously over a substantial part of the full height range corresponding to the run-up of the avalanche. A series of laboratory experiments with granular material was conducted in a 7.5 m long and 0.35 m wide chute in order to investigate impact forces on narrow rectangular and cylindrical obstacles for rapid supercritical granular flow (Fr = 13). Obstacle heights varied from about twice the flow depth to more than 20 times the flow depth, which was higher than the observed run-up, and the width of the obstacles varied from about twice the flow depth to about 7 times the flow depth. It was found that the total force on the obstacle was largely unrelated to the obstacle height for heights that exceeded about 3 flow depths, which is much lower than the run-up on the highest obstacles. For a wide range of obstacle heights exceeding about 3 flow depths, the total force on the rectangular obstacles was of similar magnitude to the dynamic pressure, 1 2 q f u 2 ; acting over an area corresponding to the width of the obstacle and the upstream depth of the flow. The total force was about 30% lower for cylindrical compared with rectangular obstacles. The variation of impact forces on obstacles with the flow velocity for thin, free-surface granular flows may be significantly different from confined flows where a free surface is not present. The presence of the free surface may lead to much lower impact forces at very high velocity than would be expected by comparison with confined flows.

RESUMEN: El diseño de descargadores de fondo es el resultado de la evaluación conjunta de múltiples aspectos. En ese contexto, la traza de un descargador de fondo aguas abajo de una compuerta podría seguir una curva, por lo que el flujo supercrítico provocará ondas cruzadas que se propagarán hasta la estructura de disipación. Dado que el túnel se encuentra confinado en altura, dichas ondas podrían interactuar con la clave modificando el patrón de ondas y sus consecuencias sobre la estructura. Este trabajo presenta una evaluación del descargador de fondo con un túnel aguas abajo de sección baúl de solera plana, con una curvatura relativa menor a 0,10 y un ángulo de 27,9°, y números de curva comprendidos entre 0,84 y 1,83, analizándose el patrón de ondas cruzadas, la incidencia sobre la clave y las condiciones de disipación mediante salto de esquí. Los resultados obtenidos muestran una incidencia importante del escalón aireador. En los escenarios con solera continua, sin aireador, se aprecia un efecto marcado de la clave del túnel sobre los niveles máximos y el ángulo para los que estos se alcanzan, siempre que el nivel alcance este sector, siendo más significativo este efecto mientras mayor resulte el nivel de embalse. ABSTRACT: The design of bottom outlet structures is the result of the joint evaluation of multiple aspects. In this context, the axis of a bottom outlet downstream a gate may follow a curve, so the supercritical flow will generate cross-waves that propagate until reaching the dissipation structure. Since the tunnel is confined in height, these waves could interact with the upper part by modifying the wave pattern and its influence on the structure. This work presents an evaluation of the outlet bottom into a trunk section tunnel of flat bottom with relative curvature less than 0.10 and an angle of 27.9°, and curve numbers between 0.84 and 1.83, being analyzed the cross-wave pattern, the impact on the upper section and dissipation conditions. The results show a significant effect of aerator. When continuous bottom was considered, an effect of the upper part of the tunnel on the maximum and the angle for those values is appreciated if water elevation goes until this sector, being more important this effect for higher reservoir levels. PALABRAS CLAVES: Flujo supercrítico, curvas, descargadores de fondo.

Two forced convection film boiling correlations are proposed for Fuel Coolant Interaction codes: one for fuel drops in premixing and explosion, and one for fuel fragments in explosion. They include features which are not always taken into account in existing correlations such as for example the Epstein and Hauser correlation: the existence of a stable and an unstable film boiling sub-regime, and inertial and convective effects in the vapor film. Also, a special care has been taken to correctly extrapolate and validate the correlation for fragments in supercritical conditions. The two correlations are built by combining a correlation for high subcoolings and a correlation for low subcoolings, which are chosen among existing or modified correlations. Regarding the film boiling sub-regimes, it is shown that when considering fuel drops both sub-regimes must be taken into account while we may assume that fuel fragments are always in the stable sub-regime. Regarding inertial and convective effects, it is shown that they should be taken into account in both correlations, and that it can be done by using the modified latent heat L' = L + 0.5CpVmΔTsup. It is also shown that using L' instead of L is essential to correctly extrapolate a correlation to supercritical conditions. Indeed, in this case, L' = 0.5CpVmΔTsup is the analogue of a pseudo-latent heat. The correlation for fragments was then compared to the TREPAM data. It was able to predict almost all data within a +/-25 % margin. Finally, in order to further validate the correlation for fragments in supercritical conditions, comparisons were made with numerical simulations with water and with experimental data with carbon dioxide. The correlation performed very well in both cases.

In a previous report, we have presented the argon nanojet injection simulation results under vacuum condition. This study also focused on the effect of injector shape on jet breakup behavior. However, geometrical effects were not so prominent. After a thorough analysis, we concluded that the temperature of the liquid in the solid injector is the governing factor for subsequent nanojet breakup characteristics. We will introduce this conclusion with detailed description in the following section. In addition, the previous nanojet injection code was modified for injection under sub/supercritical pressure. These results will also be presented later in this report.

Supercritical flow interaction occurring in the marine boundary layer between closely spaced coastal capes is investigated with a mesoscale numerical prediction model. As an extension of previous work, the U.S. Navy's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is used to perform idealized model simulations with marine layers of varying upstream Froude number to elucidate the different flow responses for a single convex bend. The impact upon the supercritical flow of introducing a series of closely spaced coastal bends is then investigated. The expansion fan is significantly reduced in magnitude and size by the formation of a compression wave at a blocking, concave bend approximately 150 km downstream. Building upon the idealized marine layer response, real-data forecasts are then examined for several time periods of supercritical flow interaction between Cape Blanco, Oregon, and Cape Mendocino, California.

To better understand the nature of the interaction between acoustic waves and liquid fuel jets in rocket engines, cryogenic liquid nitrogen is injected into a room temperature high-pressure chamber having optical access on its sides. A piezo-siren capable of generating sound waves with SPL of up to 180 dB is used under three chamber pressures of 1.46, 2.48, and 4.86 MPa. The reduced pressures for these pressures are 0.43 (subcritical), 0.73 (near-critical), and 1.43 (supercritical) respectively. The assembly consisting of the acoustic driver and the highpressure chamber form a cavity that resonates at several frequencies, the strongest being at 2700 and 4800 Hz. Three different flow rates are considered and the nature of the aforementioned interaction has been documented via a high-speed imaging system using a CCD camera. It is found that the impact of the acoustic waves on the jet structure is strongest from low to particularly near-critical chamber pressures and at low injectant flow rates. No significant effects of the acoustic waves are detected at the supercritical chamber pressure examined. It suggests that engine operation either near the critical point or under a transitory phase passing through the critical point can be troublesome and may lead or feed combustion instabilities in liquid rocket engines. Further work is needed to directly relate these effects to the observed instabilities.